Hard disk drives (HDD) use rotating disks with thin film magnetic coatings to record data. The read and write heads for each disk surface are found in a slider that is selectively positioned over the rotating disk using a rotary actuator driven by a voice coil motor controlled by a servo system. External vibrations can interfere with the drive's ability to properly write and read data in the tracks. Therefore, to improve the drive's ability to overcome problems caused by vibrations, vibration sensors can be mounted on the drive to allow the servo system to compensate for certain vibrations. A rotational vibration (RV) sensor is commonly used in disk drives. The sensors are typically mounted on the printed circuit board along with other electronic components. The mounting should be rigid to reduce sensor noise. One type of RV sensor uses two linear accelerometers oriented on different axes as linear vibration sensors. The two uniaxial acceleration sensors can detect vibration and rotation in an in-plane direction.
Rotational vibration (RV) cancellation uses detected rotational vibration to improve the position error signal (PES) for the actuator by canceling the off-track motion induced by the rotational vibration. The measured RV is input to a feed-forward control circuit that creates a feed-forward compensation signal that is summed with the control signal to the voice coil motor (VCM) for the actuator.
This method is sometimes called RV feed-forward (RVFF). The two vibration sensors should have similar gains in their primary-axis sensitivities and minimal off-axis sensitivities.
In published US patent application 20100067357 Huang, et al. (Mar. 18, 2010) describe a disk drive that compensates for rotational vibration by adaptively modifying the gains of two separate linear vibration sensors. The two sensors provide two signals to the disk drive's servo control processor which generates the control signal to the voice coil motor (VCM) that controls the positioning of the read/write head. The servo processor uses the two signals and the head position error signal (PES) as inputs to run an adaptive rotational vibration feed-forward (RVFF) algorithm to derive the control signal for the VCM actuator.
In conventional disk drives the magnetic thin films are continuous so the circular tracks are magnetic constructs created by writing magnetic transitions as the disk rotates under the head. In patterned magnetic media for disk drives, the thin film magnetic recording layer on the disk can be formed with isolated data islands that have a single magnetic domain in each island which is used to record a binary bit of data. The data island magnetic domains can consist of one grain or a small group of grains that are strongly coupled and therefore, switch together. Various methods of fabricating the data islands have been described including physical removal of the magnetic material around the islands. However, physical removal is not required. See, for example, Fullerton , et al. U.S. Pat. No. 6,383,598 which describes the fabrication of patterned magnetic recording media with regions between the islands rendered nonmagnetic by ion irradiation.
The writing of data in data islands are timed (clocked) to synchronize with movement of the data islands under the write head. In U.S. Pat. No. 6,754,017 Rettner, et al. (Jun. 22, 2004) describe a magnetic recording disk drive with patterned disk media, wherein the data islands are used as the source of the clocking signal to the write head. The slider with the read/write head also includes a special pattern sensor that senses the data islands in the tracks just before they pass beneath the write head. The pattern sensor output serves as the clocking signal to precisely control the timing of the write pulses supplied to the write head. A time delay is calculated using a timing mark on the patterned disk to delay the write pulses so that a data block sensed by the pattern sensor is the same data block to which the write pulse is applied. In this manner the previously recorded magnetic data provides the synchronization or clocking signal to control the writing of the new data.
In U.S. Pat. No. 7,675,703 Albrecht, et al. (Mar. 9, 2010) describe a disk drive with patterned media and a system for clocking write data. The precise time intervals between successive timing marks in the data tracks are measured by a timing mark detector that counts the integer number of write clock cycles between successive timing marks and the fractional part of a write clock cycle by detecting the phase difference between a timing mark and a reference signal. The resulting timing error is output to a write clock compensator. The write clock is capable of generating equally spaced primary phases and phases intermediate to the primary phases. The compensator includes a phase rotator that controls which write clock phase is selected for output. The value in a phase register of the compensator is used to control the phase rotator to advance or retard the write clock phase, and thus to adjust its frequency and phase so as to be synchronized for writing to the data blocks.
In U.S. Pat. No. 7,961,578 Hideo Sado (Jun. 14, 2011) describes a disk drive with a synchronous clock generator that inputs the rotational acceleration of the spindle motor as feed-forward control value to the feedback control system provided in the drive disk, thereby generating a clock that is synchronous with the rotational speed of the disk.
Published US patent application 20100118426 by Vikramaditya, et al. (May 13, 2010) describes a write clock control system that includes a clock controller that determines a phase offset based on a phase difference between a write clock signal and a media pattern corresponding to a given timing synchronization field being read, and a phase interpolator that produces an updated write clock signal by updating the phase of the write clock signal in accordance with control signals that are based on the phase offset signal.
In U.S. Pat. No. 6,947,243 Dang, et al. (Sep. 20, 2005) describe a disk drive that senses a rotational vibration without using any external sensor by measuring the time shift of the magnetic transitions (e.g., time jitters or bit jitters) for reference bits on the rotating disk.
Write clock compensation for write head track misregistration (TMR) from the track centerline is described in US patent application 20100309576 by Albrecht, et al. (Dec. 9, 2010). As the disk rotates, the read head detects angularly spaced servo sectors and generates a position error signal (PES) which is used by the servo control system to maintain the read head on track. As the disk rotates, the read head also detects angularly spaced synchronization marks, which are used to control the write clock so that magnetization reversal of the magnetic write field from the write head is synchronized with the position of the data islands. If there is TMR of the write head, there will be an effective shift in the timing of when the center of the data islands pass through the write field. The disk drive includes write clock phase adjustment circuitry that correlates the PES with the timing shift to compensate for TMR of the write head.
Another US patent application 20090244765 by T. Albrecht (Oct. 1, 2009) describes a patterned magnetic recording disk drive that compensates for circumferential misalignment of data island patterns among the data tracks as a result of errors in fabrication of the master template used to make the disks. To compensate for pattern circumferential misalignment (PCM) the phase of write clock for writing to the data islands is adjusted.
In U.S. Pat. No. 7,133,229 Tetsuo Semba (Nov. 7, 2006) describes a magnetic recording disk drive with patterned media with data islands and compensation for write-clock timing error caused by rotational disturbances. A rotational vibration sensor is used in conjunction with a write-clock-generation circuit to adjust the timing of the write pulses to correct for errors caused by rotational disturbances. The write-clock-generation circuit receives a reference clock signal synchronized to disk rotation and multiplies it to generate a higher-frequency write-clock signal. The write-clock-generation circuit includes a phase detector that compares the phase of the reference clock signal and the write-clock signal and provides an error signal. The output of the rotational vibration sensor is summed with the phase detector error signal to compensate for disk rotation speed changes caused by rotational disturbances.